fix(auth): normalize redirect URI URL subclasses

[he-yufeng]

Summary

Fixes #2687.

OAuthClientInformationFull.redirect_uris is declared as list[AnyUrl], but Pydantic v2 preserves already-validated URL subclasses such as AnyHttpUrl. That makes a stored AnyHttpUrl("https://...") fail membership checks against an incoming AnyUrl("https://..."), even when the URL text is identical.

This normalizes URL subclass instances back through strings at the model boundary, so redirect URI validation compares the declared AnyUrl values rather than the wrapper subclass type.

Validation

uv run pytest tests/shared/test_auth.py -q
uv run ruff check src/mcp/shared/auth.py tests/shared/test_auth.py
uv run ruff format --check src/mcp/shared/auth.py tests/shared/test_auth.py
python -m py_compile src/mcp/shared/auth.py tests/shared/test_auth.py
git diff --check